Stereoscopic displays provide an image that is made up of different sub-images at different viewing points. If suitably adjusted different sub-images (i.e. with appropriate binocular disparity) are provided to the left eye and the right eye of a viewer, the overall image is perceived by the viewer as a three dimensional image. One known method to provide differing images is by varying the colour content, with the viewer needing to wear special spectacles with a different colour-absorbing lens in each eyepiece.
Stereoscopic displays that provide an image made up of different sub-images at different viewing points without the viewer needing to wear special spectacles are known as autostereoscopic displays. A typical autostereoscopic display comprises a matrix liquid crystal display (LCD) panel comprising an array of display elements arranged in rows and columns. The display further comprises means for directing output light from the array of display elements such that the visual output provided from a given point on the display panel is dependent upon the viewing angle. This means that the right eye of a viewer will see a different view to that seen by the left eye, providing the desired stereoscopic or three-dimensional image.
One well known form of the output light directing means is a lenticular sheet overlying the display panel. A lenticular sheet, for example in the form of a moulded or machined sheet of polymer material, overlies the output side of the display panel, with its lenticular elements, comprising (semi) cylindrical lens elements, extending in the column direction with each lenticular element being associated with a respective group of two, or more, adjacent columns of display elements and extending parallel with the display element columns.
In an arrangement in which each lenticule is associated with two columns of display elements, the display panel is driven to display a composite image comprising two 2-D sub-images vertically interleaved, with alternate columns of display elements displaying the two images, and the display elements in each column providing a vertical slice of the respective 2-D sub-image. The lenticular sheet directs these two slices, and corresponding slices from the display element columns associated with the other lenticules, to the left and right eyes respectively of a viewer in front of the sheet so that, with the sub-images having appropriate binocular disparity, the viewer perceives a single stereoscopic image. In other, so-called multi-view, arrangements, in which each lenticule is associated with a group of more than two adjacent display elements in the row direction and corresponding columns of display elements in each group are arranged appropriately to provide a vertical slice from a respective 2-D (sub-) image, then as a viewer's head moves a series of successive, different, stereoscopic views are perceived for creating, for example, a greater degree of viewing freedom and/or a look-around impression.
Autostereoscopic display apparatus of this kind can be used for various applications, for example in medical imaging, virtual reality, games, mobile telephone and CAD fields.
Autosterescopic display apparatus switchable between 2-D and autostereoscopic operation are known. This is provided, for example, by provision of a diffusion layer switchable between diffusing and non-diffusing states, such that the diffusing state cancels out the light directional effect of the lenticular lens, thus reducing the autostereoscopic view to a 2-D view.
Typically, in autostereoscopic mode (or in a display having only the 3D mode), spatial resolution is lost due to the provision of separate individually addressable display elements of a given colour for different views along the row direction in a given pixel. Also, brightness variations occur across the display, i.e. in the row direction.
The effect of the spatial resolution loss in an autostereoscopic display, as discussed above, has been alleviated by the use of slanted orientation of the lenticular lens relative to the column direction of the pixels. Such an arrangement tends to “share” the loss of resolution between row and column direction, thus reducing the starkness of the resolution in the row direction, especially for larger numbers of view multi-view displays.
However, by slanting the lenticular elements at an angle to the columns of display pixels, other problems are introduced or worsened. As a user's head is moved from left to right, variations in light intensity are observed. These intensity variations are caused by the lenticular elements imaging varying amounts of the opaque black mask that defines the pixel areas of the display panel. The variations are observed by the user as moiré interference.
The problem of light intensity variations may be addressed by altering the focal axes of the lenticular elements, so that broader bands of the display panel are imaged. However, cross talk between the different views is then introduced, which is also undesirable.
U.S. Pat. No. 7,800,703 discloses an arrangement in which display areas of the display pixels have edges that are substantially parallel to the lenticular element axes. By providing a device having pixel display areas with edges that are parallel to the lenticular element axes, the problems of light intensity variations and cross talk between views are reduced or eliminated. The lenticular element axes, however, remain slanted at an angle to the display pixel columns, and so it is still possible to “consume” both vertical and horizontal resolution to increase the number views displayed by the device.
The problem of the pixel sub-structure (such as the black matrix) being visible to the viewer is a potential problem in any display apparatus having optical magnification of the display panel output. Another example of autostereoscopic display having an optical lens arrangement at the pixel output is a free focus autostereoscopic display design recently proposed by the applicant (but as yet unpublished)
In this free focus arrangement, the display system has a display device having an array of pixels and an optical system which includes a microlens array over the display device, wherein each pixel is associated with a microlens. The optical system images sub-arrays of pixels onto one pupil of a viewer, with different pixels of the sub-array imaged to different areas of the pupil. In this manner, the display intentionally images a plurality of images (one image comprising the combination of the corresponding pixels from all sub-arrays) to a single pupil.
This approach provides additional information to each pupil, in particular multiple views are provided to each pupil at the same time, and which are provided to different locations within the pupil. This provides depth information that can be interpreted by the brain. For example, even if the display is provided to one pupil only (or the same information is provided to both pupils), the brain is able to perceive a more real image because the small image differences (at the scale of the different viewing locations across the area of the pupil) encode depth information.
The optical system is preferably driven based on a pupil tracking system, wherein the light output of the combination of pixels imaged to a given area of the pupil by the array of microlenses together define an image of a scene. This provides an optical beam steering solution. This means light can be provided only to the pupil (or pupils) so creating an efficient system. However, the pupil tracking may not be required for a single-user goggle based display system. In this case, a smaller number of views can fill the field of view. The display is positioned over the eyes in a relatively constant relative position, and slight relative movement will not hinder the operation in that other views are present for the viewer. By providing different views to the two pupils, an autostereoscopic display is formed.
A trade off can be made between spatial and temporal resolution. This is especially important in applications such as TV where the number of viewers is not known beforehand. The system can serve each of the viewers (or even pupils) one by one, or serve all users a lower-resolution image, or a trade-off between the two can be made.
In this type of optical beam steering system there can be one emitter and lens system per sub-pixel, or a pixellated emitter system and lens system per pixel. With suitable parameters, the optical magnification of a beam steering display is in a typical range of 200 to 500 which is significantly larger than for a lenticular display. More specifically, the shape of the pixels will influence the bokeh (a measure of the aesthetic quality of the out of focus blur part of an image) that the viewer will observe in the out-of-focus content displayed on the screen. Thus, the pixel shape is part of the (perceptual) quality of the display.
This (unpublished) approach will not be described further. However, it will be appreciated that the problem of optical magnification of the sub-pixel structure is not limited to lenticular autostereoscopic displays, and the solutions provided by the invention are applicable to many different display designs, including the approach briefly outlined above.
Various prior art references disclose autostereoscopic display devices in which the individually addressable display elements are shaped other than rectangular. For example EP 1 929 796 discloses an arrangement in which the shape of the sub-pixels (or pixels) comprises the shape remaining from a rectangular footprint when one or more cut-outs is removed; the one or more cut-outs being positioned relative to the slanted angle of the light directing elements such that the extent by which each respective light collection line overlaps the individually addressable display elements is made more uniform.
For most display applications such as HDTV and cell phones, the pixels are small enough for the sub-pixel structure to be practically “invisible”. However if displays are combined with magnifying optical means, such as the lenticular foil in autostereoscopic displays or the free focus system outlined briefly above, the sub-pixel structure is magnified, and thus made visible to the viewer. The shaping of pixels to reduce banding, for example as outlined in EP 1 929 796 does not solve this problem.
In order to increase the efficiency and/or the lifetime of the display, all traditional displays always try to maximize the light emission per pixel by ensuring that the emission intensity across the display is at a uniform level, and ensuring that the emission area of the display is maximised (the aperture).
As a result, sub-pixels tend to have emission areas with uniform intensity, creating extremely abrupt boundaries between areas of maximum emission intensity and zero emission intensity (e.g. the black mask), combined with many right angled (90 degree) corners. For this reason, the phenomenon is clearly visible in autostereoscopic displays not only as banding but also as visibly delineated sub-pixels.